This invention relates to zeolitic materials used in elevated-temperature hydrocarbon separation and conversion processes, and relates particularly to zeolite molecular sieves. More particularly, it relates to methods for restoring such catalysts to their original activity and selectivity by removal of operating residues and impurities, and to methods of re-dispersion of metal particles for those catalysts having an outer layer of adsorbing metal.
Molecular sieves have been widely used to separate classes of organic compounds and compounds within a class. Molecular sieves are typically crystalline aluminosilicate materials belonging to the zeolite class. These materials are able to be dehydrated with very little change in their crystal structure. The dehydrated crystals possess a network of uniform cells comprising about half the total volume of crystals. The cells have a strong tendency to recapture the water molecules or any material small enough to get into the cells. This promotes a screening action making it possible to separate smaller molecules from larger ones.
When used on-stream, a hydrocarbon feedstock is passed through the catalyst at elevated temperature to accomplish the desired process of selectivity. Well-known, for example, are methods which separate n-paraffins and n-olefins from their branched counterparts; separation of aromatic from cyclic isomers; and separation of n-hydrocarbons from their positional isomers.
Molecular sieve catalysts are also widely used in hydrocarbon conversion procedures, in which, for example, selected feedstock fractions are converted into positional isomers and/or branched (skeletal) isomers of the fractions.
Also well-known, if not appreciated, is the steady buildup of impurities on the catalyst from these processes, most prominently coke, resulting from the polymerization of highly-unsaturated components of the feedstock, such as butadiene. Depending upon the specific catalyst selected and the nature of the hydrocarbon process, the point eventually comes where too much residue has accumulated for the catalyst to effectively operate. The catalyst is then subjected to a regeneration process to reduce or eliminate the coating of impurities. Total elimination is unknown, and in fact, the regeneration methods heretofor practiced produce unwanted side effects and impurities within the catalysts themselves.
For example, mordenite catalysts which are known to be used in commercial C.sub.8 - aromatic separation and isomerization processes, are usually regenerated by a high-temperature proof-burn. This is a staged burn and consists, for example, in a 700 F. degree burn with enough oxygen to raise the temperature in the catalyst about 150 F. degrees. Then the average catalyst temperature is increased by about 100 to 200 F. degrees. The disadvantages of this method include filling of the cells with impurities and moisture formed by the burning itself, and permanent damage of the zeolite structure, referred to as structural shift. The practical result of this method is a regenerated catalyst with substantially decreased activity and selectivity. This is the accepted state of affairs, however, it means that the catalyst will have to be retired after a given number of regenerations. This type of regeneration would be more effective if carried out under dry conditions; however, in the real world, it is too costly to exclude moisture from the procedure.
Most catalysts used in hydrocarbon selection methods are provided with an outer, very thin coating of a gas-adsorbing metal, typically a Group VIII metal, such as platinum or palladium. After considerable service, and particularly after coke burn during regeneration, the metal particles usually agglomerate into large crystallites. For such catalysts, a redispersion of the agglomerates is necessary to fully restore the selectivity. Two of the most common methods to achieve this rejuvenation are hot air-soaking and metal/halide complexing.
In the former process, the preferably already-regenerated catalyst is simply injected with an oxygen-in-nitrogen vapor at high temperature, for example, 900-1000 degrees F. for 2-10 hours to redisperse the metal. In the latter process, an organic halide is introduced under similar conditions. The object with all rejuvenations involving redispersants is to form metal/redispersant complexes which are then free to migrate to the sites in the catalyst structure which are in a relatively depleted condition. The main drawback of the processes is the tendency for the complexed metal to breach the zeolite cell structure and enter the internal cells. The obvious result is, of course, loss of adsorption sites and thus lower activity and selectivity of the catalyst.
The desirability of having an improved method or methods of restoring zeolite catalysts is recognized as a continuing one, and it is the principal object of the inventions disclosed herein to provide a method for regenerating a fouled catalyst, and thereafter rejuvenating a metal-containing catalyst.